The present invention was made with Government support under Grant No. DABT 63-92-C-0019, awarded by the Advanced Research Projects Agency (ARPA)/Army Research Office (ARO)/MEMS. The Government has certain rights in the invention.
The present invention relates, in general, to microelectromechanical (MEM) devices and structures, and more is particularly to high aspect ratio probes for field emitter tips, tunnelling tips, atomic force tips or the like having self-aligned gate or isolating electrodes, and to methods for the fabrication thereof.
Microelectromechanical devices utilizing micron-dimensioned emitter structures with nanometer-scale tips are exemplified by the cathode arrays illustrated in U.S. Pat. No. 5,199,917. This patent illustrates a silicon tip field emission cathode array wherein emitter tips are formed by an oxidation process and wherein precisely aligned gate electrodes are formed around corresponding emitter tips in such a way that large arrays can be formed with great accuracy and reliability. A suitable process for fabricating such single crystal silicon micromechanical structures is the single crystal reactive etch and metallization process (SCREAM) described in U.S. Pat. No. 5,198,390, the disclosure of which is hereby incorporated herein by reference.
The invention of the scanning tunnelling microscope (STM) has opened the path to atom-scale microscopy, manipulation, metrology, lithography, and spectroscopy. The STM essentially scans a metal probe over a surface to be inspected while maintaining a constant tunnelling current between the probe and the surface, thereby permitting the STM to view the surface being scanned. Later improvements of the STM included such instruments as the atomic force microscope (AFM), as well as a wide range of other related instruments. These instruments, which may generally be referred to as scanned-tip or scanned-probe instruments, each have a probe which incorporates a special tip which is sensitive to a particular nanometer-scale force or field such as a magnetic field, an electric field, a capacitance, or a van der Waals force. The basic components for a scanned-probe instrument are an appropriate tip mounted on a three-dimensional actuator to scan the tip across a surface in an x-y plane and to adjust the height (the z distance) of the tip above the surface. The instrument also incorporates a method to generate a feedback signal derived from an attached sensor to sense the height of the tip above the surface.
The basic problem with such devices, however, is that the probe tips have to be very close to a surface in order to provide the required interaction for detecting and measuring the fields or forces which exist between the tip and the surface. The prior art tip structures made it very difficult to measure rough surfaces, for the surfaces often would come into contact with the tip or the measuring instrument on which it is mounted, preventing it from accurately responding to the shape of the surface. Furthermore, in addition to measuring rough surfaces, it is often desirable to detect and to measure apertures or depressions in the target surface, and these need to be accurately and reliably measured not only as to their location, but as to their depths. However, the short height of electromechanical tips constructed in accordance with the aforementioned patents severely limits the range available for such measurements. Accordingly, there is a need for a scanning probe tip which will be capable of following surface configurations accurately and without the danger of the actuator mechanism striking the surface. For this purpose, a long, thin, needle-like structure having a high aspect ratio is needed.
In the use of scanning probes in atomic force microscopes, the probe tip is generally moved with respect to a surface being inspected. In addition, a voltage may be applied to the tip for the purpose of regulating tunnelling current flow for use in a feedback loop to control the location of the tip with respect of the surface or for the purpose of applying localized voltages to the surface for affecting the force between the tip and atoms on the surface. The interaction between the tip and the surface can be measured by the voltage between the two, with the resulting electric field lines producing forces which can be controlled by varying the height of the probe. However, a highly reliable and accurate control of the voltage as well as of the flow of tunnelling current between the tip and the surface being inspected is required for this purpose.
Since probes such as those discussed above are used to detect or measure very small fields or forces produced by highly localized features on a surface, the measurements are often subject to interference from adjacent features. Accordingly, there is a need to protect the tips from ambient fields or forces during such measurements.